The present invention relates to a method for producing a high density indium-tin-oxide sintered body.
Indium-Tin-Oxide, indium oxide-tin oxide solid solution films are utilized as a transparent electrode of a plane display material for a liquid crystal display and the like because of high conductivity and excellent transparency.
As a method for forming an indium-tin-oxide film, there are listed a method in which an indium-tin-oxide powder is applied on a substrate, a method in which an indium-tin-oxide film is formed on the surface of a substrate by sputtering method using an indium-tin-oxide sintered target obtained by molding a raw material oxide powder and sintering the molded article, as well as other methods.
According to the method in which an indium-tin-oxide powder is applied on a substrate, the resulted film has high resistance and is not used as a practical transparent electrode. Consequently, almost all indium-tin-oxide films are produced by sputtering.
Regarding the sputtering method, a method using an In-Sn alloy as a target has been conventionally used, however, the resulted film had problems in controllability and reproducibility. Therefore, a method using an indium-tin-oxide sintered body as a target is dominantly used nowadays. However, a method for producing an indium-tin-oxide film by sputtering method using an indium-tin-oxide sintered body target heretofore had a problem that there is required an operation called cleaning in which the sputtering is stopped for taking out an indium-tin-oxide target and the nodule is excised, leading to decrease in productivity because of occurrence of blackening phenomenon on the surface of an indium-tin-oxide target called nodule.
It is known that occurrence of this nodule tends to occur in a low density indium-tin-oxide target and if a high density indium-tin-oxide target is used, occurrence of nodule is suppressed, cleaning process can be abbreviated, and productivity increases consequently.
As other problems of a low density indium-tin-oxide target, there are indicated a problem that resistance increases during sputtering leading to decrease in sputtering efficiency, a problem that ash called particle grows in a sputtering apparatus, and adheres on a substrate for an indium-tin-oxide film, leading to deterioration of an indium-tin-oxide film formed and breaking of a fine indium-tin-oxide film pattern, a problem that due to high resistance of a low density indium-tin-oxide target, productivity of sputtering is low, and when power supply is increased, abnormal discharge occurs and sputtering can not conducted in stable fashion, and the like.
Therefore, various suggestions have been conventionally made for enhancing the density of an indium-tin-oxide sintered body. For example, there is a method in which a high density indium-tin-oxide sintered body is obtained by sintering under pressure using a hot press method or hot isostatic press (hereinafter, abbreviated as HIP) method. However, in the hot press method and HIP method, only a non-uniform indium-tin-oxide target is obtained because tin oxide can not be made into sufficient solid solution and the resistance of an indium-tin-oxide target can not be decreased since temperature can not be increased in the method. Further, when the hot press method or HIP method is used, the price of a product increases due to an expensive equipment required.
There is also suggested a method in which indium-tin-oxide powders are sintered in a pressured oxygen gas atmosphere (Japanese Patent Application Laid-Open (JP-A) No. 3-207858), however, it has a problem that the price of a production is high since an expensive special equipment which can endure pressure is required for conducting sintering in a pressured oxygen gas atmosphere. Further, it includes problems regarding safety of a high pressure oxygen gas atmosphere, and the like.
There is also suggested a method in which indium-tin-oxide powders are sintered in an oxygen gas atmosphere under an atmospheric pressure (Japanese Patent Application Laid-Open (JP-A) No. 6-299344), however, it has a problem that a high density ITO sintered body can not be obtained.
Furthermore, there is also suggested a method in which a tin oxide powder having larger diameter is mixed with a fine indium oxidepowder (JP-A No. 6-183732), however, this method is not necessarily sufficient for achieving high density.
The object of the present invention is to provide a high density indium-tin-oxide sintered body which is excellent in property as a sputtering target and is industrially useful, in a method for producing an indium-tin-oxide sputtering target comprising molding an indium-tin-oxide powder and then sintering the molded one.
The present inventors have intensively studied for solving the above-described problems, and have found that increase in the density of the final sintered body is promoted when the partial pressure of moisture in an indium-tin-oxide sintering atmosphere is lower or the concentration of oxygen in an indium-tin-oxide sintering atmosphere is higher, and have completed the present invention.
Namely, the present invention provides a method for producing an indium-tin-oxide sintered body comprising the steps of;
molding a powder comprising indium, tin and oxygen, and
sintering the molded article in oxygen gas-containing atmosphere,
wherein the halogen content of the powder comprising indium, tin and oxygen is 0.02% by weight or less, the concentration of oxygen in the oxygen gas-containing atmosphere is 90% or more during the sintering, the partial pressure of moisture contained in the oxygen gas-containing atmosphere is 800 Pa or less during the sintering, and the molded article is maintained at a temperature in the range from 1500 to 1650xc2x0 C. for 1 hour or more for the sintering.
The present invention will be described in detail below.
As a powder comprising indium, tin and oxygen, a powder comprising indium, tin and oxygen which can be produced according to a method for producing an indium-tin-oxide powder described in JP-A No. 10-72253, a mixed powder of an indium oxide powder and a tin oxide powder, a mixed powder of an indium-tin-oxide powder and tin oxide powder, a mixed powder of an indium-tin-oxide powder and indium oxide powder and the like are listed. Hereinafter, these powders are abbreviated and called ITO or ITO powder in the present invention. These are made into molded articles by the following methods.
Said powder can be optionally selected so that the content of tin oxide in an ITO sintered body preferably from 1 to 50% by weight, more preferably from 2 to 20% by weight.
Said powder preferably does not contain halogen, and the halogen content of the powder is preferably not more than 0.02% by weight.
It is preferable to use said powder having a primary particle size calculated by BET specific surface area of the powder of preferably from over 0.05 xcexcm to 1 xcexcm or less, more preferably from over 0.1 xcexcm to 0.5 xcexcm or less.
It is preferable to grind said powder since the secondary particle may sometimes be larger due to coagulation. The central particle size calculated by particle size distribution after grinding is preferably not over 1 xcexcm.
As the grinding method, dry ball mill grinding, wet ball mill grinding, jet mill grinding or combination thereof can be used.
By using such preferable said powder, a high density ITO sintered body can be obtained which has a sintered body density of 7.10 g/cm3 or more and compacted to 99% or more of the true density (the true density of ITO is 7.16 g/cm3).
Then, said powder is molded into desired form, further sintered to obtain an ITO sintered body.
As a method for obtaining a molded article of said powder used in the present invention, various methods can be used, and there are listed, for example, dry press, cool isostatic molding press (CIP) and the like as the dry molding method, slip casting, deep casting and the like as the wet molding method, other injection molding methods and the like. In the case of the dry molding method, if a powder is previously made into a slurry and then made into a granule by spray dry and the like, such effects are obtained that operability in molding increases, a more uniform molded article is obtained, and the like.
In the molding, a dispersing agent for enhancing dispersibility of a powder, a binder for imparting shape keeping property, a releasing agent of easy unloading from a mold, a lubricant for reducing mutual friction of powders, and the like are added if necessary. As the dispersing agent, nonionic, cationic surfactants and the like, as the binder, polymers such as polyvinyl alcohol, polyethylene glycol, polymethylmethacrylate and the like, as the releasing agent, micro wax emulsion and the like, as the lubricant, glycerin and the like, are suitably used.
Thus obtained molded article of said powder is sintered and compacted. The sintering is an operation for mutual adhesion of powders by way of diffusion and requires high temperature, but can be conducted using a usual sintering apparatus such as an electric furnace and the like.
In sintering, the molded article is maintained at a temperature in the range from 1500 to 1650xc2x0 C. for 1 hour or more, preferably 3 hours or more.
Before sintering, the molded article is preferably maintained at a lower temperature than that in initiation of the sintering, namely at a temperature in the range from 1000 to 1300xc2x0 C. for 1 hour or more, further, 3 hours or more.
In the present invention, sintering is preferably conducted in an oxidizing atmosphere, namely, an oxygen gas-containing atmosphere, air having enhanced oxygen concentration, oxygen atmosphere and the like are listed and an atmosphere having an oxygen concentration of 90% or more is preferable, and an atmosphere having an oxygen concentration of 95% or more is more preferable. It is preferable that an oxygen gas-containing atmosphere in sintering contains no moisture, and regarding the moisture content, the partial pressure of moisture is 800 Pa or less, preferably 400 Pa or less, further preferably 200 Pa or less.
For reducing the moisture content of an oxygen gas-containing atmosphere in sintering, it is preferable to removed the remaining moisture and moisture adhered to a furnace wall, furnace material, heat insulation material and the like, by evacuation inside of an electric furnace. It is also preferable to reduce the amount of moisture in the atmosphere in an electric furnace by flowing an oxygen gas having low moisture content such as an oxygen gas filled in an oxygen bomb. There is also a method to reduce the amount of moisture dissolved in a gas by passing the gas through a desiccant and the like.
According to the method for producing an ITO sintered body of the present invention, a high density ITO sintered body is obtained, and when the instant sintered body is used as a sputtering target, reduction of nodule, improvement in sputtering efficiency, decrease in a particle, reduction of abnormal discharge, enhancement of quality of a formed ITO film, and the like can be achieved.
A compact high density ITO sintered body has excellent property as an ITO sputtering target and industrially useful.
A sputtering target for manufacturing a transparent conductive film such for a liquid crystal display (LCD) and the like is obtained by a usual method for producing an ITO target in which an ITO sintered body of the present invention is cut and polished, and bonded to a copper backing plate by indium solder.